A. Technical Field
The field involves device fabrication in which material is selectively removed by plasma-assisted etching. A significant category of devices is silicon integrated circuitry with sufficient integration to yield large scale integration or even very large scale integration.
B. History
Device design trend toward increasing miniaturization has resulted in the need to define features of the order of micrometers and less. This trend is resulting in replacement of the liquid etch processes so useful for small and medium scale integration with varieties of dry processing.
Dry processing so desirable for a variety of reasons--e.g., relative freedom from condensed phase contaminant--often takes the form of plasma-assisted etching.
Plasma-assisted etch processes are identified by various designations--e.g., plasma etching, reactive ion etching, etc. With the exception of ion milling which is normally practiced with argon or other inert species, some considerable part of material removal is due to chemical reaction (as distinguished from momentum transfer).
Such plasma-assisted etch processes generally depend upon a plasma-derived species which acts as oxidant for the material to be removed. Typical oxidants are atomic chlorine and atomic bromine.
Plasma compositions commonly include other species. Additional species may be or may yield chemically unsaturated compounds or radicals. This additional species may be unsaturated as introduced into the plasma or may be derived from freon or some compound of higher oxidation state by action of the plasma. Unsaturated material may not be deliberate but may be a by-product--e.g., of etchant generation. Accordingly, a freon may be introduced to yield monatomic fluorine etchant with such precursor contributing unsaturated radical, as well. In other instances, a precursor compound yielding unsaturate is introduced to control etch profile and/or to minimize "loading" (etch rate dependence upon surface area revealed). See, for example, "The Loading Effect in Plasma Etching" by C. J. Mogab, Journal of the Electrochemical Society, Vol. 124, No. 8, (August 1977).
A necessary consequence of unsaturate presence is etchant species consumption. Etchant species consumed by combination with unsaturate necessarily results in reduction of etch rate.